The idea of using endogenous skeletal muscle as an energy source for cardiac assistance is not new. Many investigators have demonstrated that untrained skeletal muscle can provide circulatory support for short periods of time (minutes to hours). However, early studies were plagued by rapid muscle fatigue which discouraged the use of skeletal muscle as a long-term energy source for biomechanical assistance. This ultimately prompted researchers to explore the possibility of conditioning skeletal muscle via chronic electrical stimulation.
The ultimate objective of chronic myostimulation is to transform muscle to a fatigue-resistant state so that it performs much like the myocardium, which is an oxidative muscle capable of continuously pumping blood. Both cardiac and skeletal muscle contain contractile proteins which transform chemical energy into mechanical work, but skeletal muscle comprises several types of muscle cells with different physiologic and metabolic characteristics. These contractile fibers may be either glycolytic (fatigue-susceptible) or oxidative (fatigue-resistant). Slow-twitch muscle is generally oxidative, while more powerful fast-twitch muscle can be either glycolytic or oxidative. In order to utilize skeletal muscle for long-term circulatory assist, a conditioning scheme is needed to convert these fibers from glycolytic to oxidative metabolism.
The feasibility of converting fast-twitch muscle fibers to fatigue-resistant slow-twitch fibers was demonstrated by Salmons and Sreter in the mid-70s. (Salmons, S., and Sreter, F. Significance of impulse activity in the transformation of skeletal muscle type. Nature, vol. 263, 30-34, 1976). Since then, interest in skeletal muscle conditioning has increased and has become a major subject of current studies. (Sreter, F., Pinter, K., Jolesz, F., and Mabuchi, K., Fast to slow transformation of fast muscles in response to long-term phasic stimulation. Experimental Neurology, vol. 75, 95-102, 1982; Macoviak, J., Stephenson, L., Armenti, F., Kelly, A., Alavi, A., Mackler, T., Cox, J., Palatianos, G., and Edmunds, L., Electrical conditioning of in situ skeletal muscle for replacement of myocardium. J. Surgical Research, vol. 32, 429-439, 1982). Recently, both Frey and Dewar have shown that a short train or "burst" of pulses is effective in producing a sustained contraction with complete conversion of skeletal muscle to fatigue-resistant fibers. (Frey, M., Thoma, H., Gruber, H., Stohr, H., Huber, L., Havel, M., and Steiner, E. The chronically stimulated muscle as an energy source for artificial organs. Eur. Surgical Research, vol. 16, 232-237, 1984; Dewar, M., and Chiu, R. Cardiomyoplasty and the pulse-train stimulator. Biomechanical Cardiac Assist, Futura Publ. Co., 43-58, 1986).
This knowledge has led to a myriad of new techniques designed to utilize the transposition of conditioned contractile tissue. Applications include cardiomyoplasty, diastolic counterpulsation, and using the muscle as an energy source to drive cardiac assist devices. To date, most attempts to harvest this new power source have involved wrapping a muscle flap around the heart, aorta or prosthetic conduit, or shaping the muscle into a neo-ventricle. (Magovern, G., Park, S., Kao, R., Christlieb, I., and Magovern, Jr., G. Dynamic cardiomyoplasty in patients. J. Heart Transplantation, vol. 9, 258-263, 1990; Pattison, C., Cumming, D., Williamson, A., Clayton-Jones, D., Dunn, M., Goldspink, G., and Yacoub, M. Aortic counterpulsation for up to 28 days with autologous latissimus dorsi in sheep. J. Thoracic Cardiovascular Surgery, vol. 102, 766-773, 1991; Mannion, J., Hammond, R., and Stephenson, L. Hydraulic pouches of canine latissimus dorsi; potential for left ventricular assistance. J. Thoracic Cardiovascular Surgery, vol. 91, 534-544, 1986; Hammond, R., Bridges, C., DiMeo, F., and Stephenson, L. Performance of skeletal muscle ventricles: effects of ventricular chamber size. J. Heart Transplantation, vol. 9, 252-257, 1990).
These efforts have met with only limited success because of insufficient power generation, slow relaxation, muscle atrophy, and thromboembolism. Wrapping the muscle flap is an inefficient means for collecting muscle energy and requires complete mobilization of the muscle, which interrupts some of its blood supply and increases the development of fibrosis. It is apparent that muscle-wrapping techniques are not the best way to harvest useful work from skeletal muscle, and that alternative pumping schemes should be explored.
Previous studies have demonstrated that certain large skeletal muscles can produce high amounts of aerobic (steady-state) energy if the muscle is left in situ and allowed to contract linearly. For example, Ugolini recently published an article describing an energetic balance of the human psoas major that predicts a steady-state power capacity of 5.19 watts. (Ugolini, F. Skeletal muscle for artificial heart drive: theory and in vivo experiments. Biomechanical Cardiac Assist, Futura Publ. Co., 193-210, 1986). This power level would be more than sufficient to drive a hydraulic artificial heart (which has a typical power requirement of&lt;1.2 watts) if a means could be devised to efficiently convert this energy into pumping power.
Logically, one of the most effective ways to harness muscular work for this purpose would be to employ a linear arrangement with the muscle tendon detached from its original insertion point and reconnected to a hydraulic energy converter. This hypothesis has recently been tested by Sakakibara (Sakakibara, N., Tedoriya, T., Takemura, H., Kawasuji, M., Misaki, T., and Watanabe, Y. Linear muscle contraction for actuation of a muscle-powered cardiac assist device (MCAD): an ex vivo pilot study. ASAIO Abstracts, vol. 21, p. 53, 1992), who evaluated the performance of latissimus dorsi (LD) muscle in three orientations: roll (wrap) type; compressive type; and linear type. This study showed that linear actuation produced a six-fold improvement in work output versus wrap-type, and a 50% increase over the compressive arrangement.
Another study, performed by Geddes at Purdue University (Geddes, L., Badylak, S., Tacker, W., and Janas, W. Output power and metabolic input power of skeletal muscle contracting linearly to compress a pouch in a mock circulatory system. J. Thoracic and Cardiovascular Surgery, vol. 104, 1435-1442, 1992), utilized three different groups of muscles (contracting linearly) to compress a valved pouch in a hydraulic model of the circulation. Output power and metabolic input power were measured at contraction rates from 10 to 40 per minute. Muscle blood flow increased dramatically during periods of work, and fatigue was not a factor even though unconditioned muscles were used. Continuous power generation of over 2 watts was recorded for all groups, with an energy conversion efficiency approximating that of cardiac muscle (10%). Based on these data, Geddes concluded that an energy-conversation scheme should be sought in which linear shortening of skeletal muscle could be used to assist the circulation.
The concept of powering a pump with linearly contracting muscle appeared in the literature as early as 1964, when Kusserow and Clapp employed a quadriceps femoris muscle to drive a spring-loaded diaphragm pump. (Kusserow, B., and Clapp, J. A small ventricle-type pump for prolonged perfusions: construction and initial studies, including attempts to power a pump biologically with skeletal muscle. Trans. ASAIO, vol. 10, 74-78, 1964.) Since that time, a number of investigators have addressed this topic, yet no serious attempts to develop such a device were published until Sasaki in 1992. (Sasaki, E., Hirose, H., Murakawa, S., Mori, Y., Yamada, T., Itoh, H., Ishikawa, M., Senga, S., Sakai, S., Katagiri, Y., Hashimoto, M., Fuwa, S., and Azuma, K. A skeletal muscle actuator for an artificial heart. ASAIO Journal, vol. 38, 507-511, 1992.) His system employs a flexible rod, sheath, crank, and cam to transmit muscle power to a pusher-plate pump. This scheme was tested in dogs using an untrained latissimus dorsi muscle and a mock circulatory system. At 60 beats per minute, this device maintained 0.8-2.0 L/min for 200 minutes against an afterload of 75 mmHg. Output power was 2.5 mW/gram, and system efficiency approached 50%.
Most recently, Farrar and Hill developed a new muscle-powered, linear-pull energy converter for powering various implantable devices. (Farrar, D., and Hill, J. A new skeletal linear-pull energy convertor as a power source for prosthetic circulatory support devices. J. Heart and Lung Transplantation, vol. 11, 341-350, 1992.) This device consists of a metal cylinder which houses a piston-type actuator to which the muscle is attached. As the muscle contracts, up to 2 ml of hydraulic fluid is displaced into a transmission line leading to the blood pump. During relaxation, a spring is employed to assist pump filling and muscle lengthening. This device has problems, though. For instance, energy convertor components were purchased off the shelf, and no attempt was made to minimize energy losses. Consequently, the efficiency of this device was found to be only 22.5%.
Clearly, interest in muscle-powered assist devices is on the rise. However, efforts to date have produced systems that employ conventional mechanisms subject to frictional losses and component wear. Not only does this limit the lifetime of the prosthesis, but reduces the amount of work obtained from the muscle. Given the limited aerobic capacity of skeletal muscle, future research must focus on optimizing system efficiency in order to ensure long-term operation and effective muscle use.
Despite recent advances in both pharmacologic and surgical therapies for heart failure, about 250,000 Americans die each year from this condition. A totally-implantable ventricular assist device would be an effective treatment, but the complexities of transcutaneous power delivery and problems associated with blood contacting surfaces have severely limited this approach. The hydraulic ventricular assist device was developed as a means to utilize muscle power to aid the failing heart while avoiding direct contact with the bloodstream.
This method of cardiac assistance offers an attractive alternative to current long-term support schemes by eliminating the need for artificial power sources. Through this mechanism, external battery packs, power conditioning hardware, transmission coils, and internal power cells could all be replaced by natural biomechanical processes, serving to greatly enhance patient quality of life by improving reliability, eliminating external components, and reducing costs. Moreover, external cardiac compression avoids critical problems associated with man-made blood contacting surfaces (e.g., clotting and hemolysis).